chore: enforce PLC0415 in tests for the remaining packages (#8547)

Follow-up to #8540, which turned on `PLC0415` (import-outside-top-level)
for checkpoint-postgres and checkpoint-sqlite. This does the remaining
six packages: checkpoint, checkpoint-conformance, langgraph, prebuilt,
cli, sdk-py.

Scoped to tests, per @sydney-runkle's call on #8540: library code is
exempted with `per-file-ignores`, since it still has deferred imports
nobody has reviewed and mixing that in would make this hard to read.

## What changed

Function-level imports across 56 test files moved to module level. Nine
could not move and carry an explicit `# noqa: PLC0415` with a reason:

| File | Why it stays local |
|---|---|
| `libs/langgraph/tests/test_deprecation.py` (4) | the import has to run
inside `pytest.warns` for the warning to be observed |
| `libs/langgraph/tests/test_serde_allowlist.py` | try/except guard,
skips when langchain_core is absent |
| `libs/langgraph/tests/test_delta_channel_benchmark.py` | optional
psycopg probe |
| `libs/checkpoint/tests/test_conformance_delta.py` (3) | protected by a
module-level `pytest.importorskip`; hoisting past the guard turns a skip
into a collection error |

That last one is the trap: an import moved above `pytest.importorskip`
silently defeats the guard. I hit it locally and it turned the skip into
a `ModuleNotFoundError` at collection. Every file with an `importorskip`
or `except ImportError` was checked by hand for this.

## Verification

`make lint` and `make test` in each of the six:

| Package | Tests |
|---|---|
| checkpoint | 156 passed, 17 skipped |
| checkpoint-conformance | 1 passed |
| langgraph | 1968 passed, 4 skipped |
| prebuilt | 284 passed |
| cli | 336 passed |
| sdk-py | 493 passed |

Also confirmed the rule actually fires: a throwaway test file with a
function-level import is flagged in all six packages, and the source
exemption holds.

libs/cli/tests/unit_tests/test_deploy_helpers.py

@@ -347,7 +347,6 @@ class TestCallHostBackendWithOptionalTenant:
 
     def test_workspace_prompt_blocked_by_no_input(self, monkeypatch):
         """With _no_input=True, 403 requiring workspace should raise ClickException."""
-        import langgraph_cli.deploy as deploy_mod
 
         monkeypatch.setattr(deploy_mod, "_no_input", True)
 
@@ -515,7 +514,6 @@ class TestEmitterTextMode:
 
 class TestCreateHostBackendClientNoInput:
     def test_raises_when_no_api_key_and_no_input(self, monkeypatch, tmp_path):
-        import langgraph_cli.deploy as deploy_mod
 
         monkeypatch.setattr(deploy_mod, "_no_input", True)
         monkeypatch.delenv("LANGSMITH_API_KEY", raising=False)
@@ -530,7 +528,6 @@ class TestCreateHostBackendClientNoInput:
             )
 
     def test_succeeds_with_api_key_in_env(self, monkeypatch, tmp_path):
-        import langgraph_cli.deploy as deploy_mod
 
         monkeypatch.setattr(deploy_mod, "_no_input", True)
         monkeypatch.setenv("LANGSMITH_API_KEY", "lsv2_test")